Swing low may be the chant of another country, but Wales went lower and lower to scythe down Ireland's powerful runners. They turned mauls into rucks, won the battle of the ball on the floor and played at such a sustained pace throughout that the men in green lacked the puff to stage a late fightback.

Where Ireland had kept Australia's ball-carriers upright last month, mauling and scrummaging their way to victory, they were never able to impose themselves on a resourceful Wales, who thoroughly merited their second World Cup semi-final, 24 years after their first, which was also held in New Zealand.

It was a victory, above all, for the Wales coach, Warren Gatland. When the Welsh Rugby Union agreed a four-year contact extension with him 11 months ago, they received few plaudits. The team was perceived as being in decline after the 2008 grand slam success and devoid of inspiration. Gatland said before the start of this year's Six Nations that he was reversing his policy of empowering the players. He felt they responded better when told exactly what to do and, as was the case when he took over as coach in 2007, he is firmly in charge.

Wales have not only surprised opponents with their conditioning, a legacy of two harsh training camps in Poland in July that pushed the players to the limits, physically and mentally, but their tactics have been smart. They named Shane Williams on the left wing for the quarter-final, only to switch him with George North and deploy him on the right, reckoning Ireland would have spent the week working on weighted kicks to Williams for the taller Tommy Bowe to chase, a tactic that helped them win the 2009 grand slam in Cardiff.

Instead, Bowe found himself up against the rather more substantial North, while Williams was opposite a player more his own size, Keith Earls. Before Ireland had the chance to come to terms with the switch, they were seven points down: two minutes had gone when Jamie Roberts, having already claimed a high ball from Rob Kearney, left Donncha O'Callaghan checking his head was still attached to his shoulders after running into the second row with the force of a runaway bulldozer. Two phases later, Leigh Halfpenny gave a try-scoring pass to Williams in the right-hand corner. It was exactly the same moment, and in virtually the same spot, that South Africa took a seven-point lead in Wales's opening group match here last month, and if it inspired Ireland to respond forcefully, they were shunted sideways by Wales's tackling technique that allowed even Williams and Halfpenny to mow down marauding forwards by grabbing their ankles.

Ireland managed a driving maul from a lineout, but only once in the match did they earn a scrum by preventing a Wales ball-carrier from going to ground. Otherwise, it was a battle on the floor and Ireland had no one to compete with Sam Warburton and Dan Lydiate. The selection of Ronan O'Gara did not work. His kicking game was awry and he lacked the understanding with his half-back partner, Conor Murray, that Rhys Priestland had with Mike Phillips. Ireland went into the tournament without a clear first choice at scrum-half, having left Tomas O'Leary at home, and they could have used the experience of Peter Stringer.

Phillips was inspired for Wales, four months after his international career was in the balance. He was dropped from the squad after a drunken night out in the centre of Cardiff found its way on to the internet and he was only readmitted after Gatland had warned him that one more infraction would see him excluded permanently. Phillips has responded with the form of his life. He scored Wales's decisive second try after Ireland had fought back to 10-10, first through an O'Gara penalty – which was poor reward for 10 minutes of pressure in and around the opposition 22, when Rory Best appeared to be everywhere, but too many receivers were static – and then with a try by Earls, four minutes after the restart. This came after Ireland created so much space that not even poor passes from Murray and Bowe could deny the wing a clear run to the line.

Wales had gone into the interval 10-3 ahead after Halfpenny had kicked a penalty from just inside the Ireland half, into the wind. The second row, Luke Charteris, who had made 14 tackles, failed to reappear after the break, having suffered a neck injury, and Wales's defence initially stuttered in his absence – but, at the moment they appeared to have surrendered control, Phillips imposed himself. He was ticked off, on the recommendation of the touch judge, Wayne Barnes, for waving his arms around at rucks, but he was waving while Ireland were drowning.

Little appeared to be on when he took the ball from a ruck 10 metres from the Irish line, but he noticed Murray was on the floor and that Gordon D'Arcy, who was guarding the blindside, had moved closer to the breakdown. Phillips moved into the space D'Arcy had vacated, handed off the centre after he belatedly saw the danger and shrugged off a challenge by Bowe to score in the corner. Unlike Phillips's match-winning try against Ireland in this year's Six Nations, there was no disputing its legitimacy.

Ireland were by now feeling the effects of the battering they had taken from the likes of Phillips, Roberts, who was not deterred by a rearranged nose and kept thumping into challenges, North and Toby Faletau. The conditions had seemed made for Ireland, a strong wind and the occasional heavy shower, but Wales tailored their game, eschewing off-loading, kicking out of hand more often than Ireland and making fewer passes.

Priestland hit the post with the conversion attempt of Phillips's try and then missed a 35-metre penalty, but Wales never lost their focus. Adam Jones had gained the measure of Cian Healy up front, twice winning penalties, and with the forwards ensuring quick ball from the breakdown, Wales applied the coup de grace 14 minutes from time.

Ireland were slow to react when Priestland changed direction as he waited for the ball from a ruck. He looped to the right and passed to Jonathan Davies, 30 metres out. As Earls dithered, the centre shrugged off three challenges to force his way over the line. Priestland hit the post with a penalty six minutes later that would have killed the game, mathematically, but Ireland were spent and Wales were on their way to Auckland and a semi-final against France.

Asked about his future, having been linked with the New Zealand job, Gatland replied: "I am staying with Wales, unless they decide to sack me." Wales's only other semi-final appearance came in the only other World Cup when they had not sacked their coach in the buildup to the tournament. Their reward is Les Bleus rather than the blues.

This is an updated match report for the Observer and replaces guardian.co.uk's earlier version